Cracking up

Potassium dihydrogen phosphate, KH2PO4 (KDP), its fully deuterated analog, DKDP, and other members of this compositional family are technologically important electro-optic crystals. These materials are widely used in high-power laser systems for electro-optic switching, polarization smoothing, and for nonlinear optical frequency conversion (as frequency doublers and triplers). Large crystals, with a radius of about 1 m, can be commercially grown from solution. Pure KDP exists in a paraelectric tetragonal phase from about −150 °C to 210 °C [1], but this range narrows considerably with increasing deuterium content, with fully deuterated materials being stable from about −50 °C to 140 °C [2]. A monoclinic phase is formed at the upper end of the temperature stability range, while at the lower end, a ferroelectric orthorhombic phase forms. It has also been reported that chemical decomposition of KDP with the release of water starts at about 180 °C [1].

The surfaces of transmitting optical components in high power laser systems require coatings to reduce their reflectivity and the attendant loss. The simplest interference antireflective (AR) coating consists of a single quarter-wave layer of transparent material whose refractive index is the square root of the substrate's refractive index. This provides zero reflectance at the center wavelength and decreased reflectance for surrounding wavelengths. Depositing coatings on KDP and DKDP materials is challenging due to the limited temperature stability range of the tetragonal phase and the fragility of these materials. KDP has a toughness that is less than half that of optical glasses [4] and large anisotropic coefficients of thermal expansion [5]. This makes the material extremely intolerant to thermal-shock, and small thermal gradients can lead to mechanical failure. Thus, processes such as physical vapor deposition (PVD) which cause heat generation in the sample can only be used with extreme caution. Even if dielectric coatings are deposited successfully, the CTE mismatch between the substrate and coatings has a profound impact on the reliability of the coatings. Consequently, partially dense sol-gel based coatings have been developed for use on large optics made of this material [3].

We are critically examining the reported phase stability behavior of the KDP family of materials. While the lower limits of the phase change temperatures for KDP and DKDP are well established, there is considerable debate about the physical processes that occur at the upper limits (for KDP) [1]. This lack of clarity is due to the overlap between the phase change temperature and the range over which dehydration occurs. Similar studies have not been attempted for DKDP.

Using x-ray diffraction, thermal dilatometry, and Raman spectroscopy in combination with in situ optical and scanning electron microscope (SEM) observations, the dehydration, phase change, and melting behavior of several representative compositions is being characterized. We intend to fully characterize the decomposition behavior, so that the phase stability bounds can be established with greater confidence. As sol-gel based coatings are difficult to apply uniformly for small components, and the stability of these coatings is in question, we are also investigating the reliability of PVD coatings on KDP materials.

The reflected light micrograph shown on this month's cover is an example of the extreme fragility of coatings on DKDP. The material was coated with a quarter wavelength MgF2 AR coating using a PVD process. The sample, formerly at room temperature, was inadvertently placed on a cool glass plate, which was ∼ 20 °C cooler. This thermal shock induced a biaxial tensile strain in the near surface of the sample, and this tension led to coating fragmentation. SEM examination of the sample revealed that small areas of the fragments were still attached to the substrate, while the rest of the fragment had curled up. The curling of the film indicates that its volume has increased after the partial release, which means that the as-deposited film was under compression. The region near the edge did not crack, which could be due to the coating being thinner in that area. Interference of the incident light with these curled fragments generates the aesthetically pleasing colors seen in the photograph.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04–94AL85000.

Further Reading
[1] J.A. Subramony et al. Chem Mater, 11 (5) (1999), p. 1312
[2] Tandon, R. and Swiler, T.E., unpublished work.
[3] I.M. Thomas, Appl Optics, 25 (9) (1986), p. 1481
[4] T. Fan, J.C. Lambropoulos, J Am Ceram Soc, 85 (1) (2002), p. 174 
[5] J.S. Browder, S.S. Ballard, Appl Optics, 16 (12) (1977), p. 3214